HI Joe,
What's a reasonable loss? If your going without a post mixer filter (no
post mixer gain) then it should be low.
If gain can be added there (between mixer and filter) than any loss
becomes tolerable. Granted that may cost
the IP3 on RX but makes other things a bit easier.
Allison
On 5/21/15 12:31 PM, Joe Rocci wrote:
Hi Steve
You might have discovered the same ones I did about a year ago. I built a filter with them and also with some 30 Mhz ones too. I posted one or both here, maybe the 30 mhz one. The problem I ran into with the 40 mhz xtals was that, to get reasonable loss, you had to make the bandwidth upward of about 6 khz. I’ll see if I can find the plots I did.
Joe
W3JDR
*From:* Steven B. Dick <mailto:sbdick@xxxxxxxxxxxxx>
*Sent:* Thursday, May 21, 2015 12:23 PM
*To:* minima@xxxxxxxxxxxxx <mailto:minima@xxxxxxxxxxxxx>
*Subject:* [minima] Re: something fresh - a long long post
Great job!! In order to preserve your beloved 10 meter band, I came across some fundamental mode 40 MHz crystals made by ECS, part number ECS-400-20-3X-TR. See Digikey: http://www.digikey.com/product-detail/en/ECS-400-20-3X-TR/XC1786CT-ND/2676650 costing only 56 cents, quantity one dropping further with quantities. Could be used to make a 4 pole quasi equiripple filter, as you like to use.
I purchased some but have not been able to get to characterizing them yet as I have so many projects ahead of it. They have a specified ESR of 40 ohms. Not great, but could possibly be used to allow 10 meter operation.
Regards, "Digital Steve", K1RF
------------------------------------------------------------------------
*From:* minima-bounce@xxxxxxxxxxxxx [mailto:minima-bounce@xxxxxxxxxxxxx] *On Behalf Of *Ashhar Farhan
*Sent:* Wednesday, May 20, 2015 3:14 PM
*To:* minima@xxxxxxxxxxxxx
*Subject:* [minima] something fresh - a long long post
comrades,
i know that it has been a very long time since i posted anything substantial on this list. but i have been quite active measuring, thinking and trying out various things for the minima. I have finally arrived at some conclusions that i'd like to present to you all.
I have produced a new version of the Minima that is substantially simplified, easy to work with. However, it comes at two major trade-offs (that is why we are engineers, not scientists : we work to a budget). First, the transceiver works from DC to 21 MHz. I had to drop 10 meters - a personal favourite. Second, I have switched to a diode mixer resulting in a drop of IIP3 performance down to around +15dbm. This is still superb. But not in the same league as before.
Finally, I have spent the last two days using the rig. It is a really sweet sounding transceiver. Easily the best I have used. The circuit is simplified to the extreme. It is even simpler than the BITX.
Here is my long story about it :
Measurements
I realized that I didn't have the equipment to actually test and measure IIP3, loss, etc. Hence, I spent a few months building equipment. I now have a spectrum analyzer, entirely home-built that has excellent dynamic range. It is based on the same Arduino + Si570 combo as the 1st oscillator. The rest is an evolution of the W7ZOI's spectrum analyzer. Along the way I learnt to sweep VHF filters, and measure IIP3. This analyzer can step in 1 Hz steps (thanks to you guys for having developed a better Si570 library for radiono) and I have a narrow 500 Hz and wide 300 KHz filters. But that is an entirely separate topic for another article.
I also made a two-oscillator setup by pulling 14.318 MHz crystals apart by 20 KHz. And combined their buffered output in a 6db hybrid combiner and took the output through a an LPF cut for 14 MHz. Thus I had the ability to measure loss, intercept, band-pass. I still lack the ability to measure noise figure due to a lack of calibrated noise source.
Equipped with this, I set out to hack the KISS mixer. I have spent a great deal of time trying to build them with discrete devices. I tried everything : from 2N3904s through 2N7000s to J310s. I tried five different biasing schemes. I have documented it all in my notes. The summary is simple : the KISS mixer lacks enough suppression of the LO to be used in the middle of a passband. It is an excellent mixer for high performance receivers. One could add some narrow band filters to the Minima and a Linrad backend to beat the living daylights out of K3S. Btw, I measured more than 30 dbm IIP3 on the KISS mixer, original version. I say 'more than' because my -10dbm per tone signal source was hitting the noise floor on the specan.
1. Giving KISS a miss
So, the KISS mixer has to be parked aside for the a minmal rig like the MInima. That leaves us with the old favourite : the diode ring mixer. I built a diode ring mixer with 1N4148 diodes that measured 15dbm IIP3. This can be as good as any of the higher performance rigs. (http://www.elecraft.com/K2_perf.htm#Main RX Table)
However, the standard mixer circuit took the IF from the center tap of the tranformer that was driven by the LO. This leaked the LO to the IF (which we use as the RF port). By grounding the center tap of the LO transformer and taking the IF from the center tap of the other transfomer (the one connected to the RF port), the LO dropped substantially. It went down by almost 57-60dbc (below the carrier).
The diodes will have to be matched to the last millivolt : easily done with a two dollar DVM.
2. Dropping 10 meters
I had written earlier that i was fooling around with 24 MHz crystals. These commonly available and inexpensive too. By moving the IF to 24 MHz, we achieve a number of things. First, an 4 section LPF cut for 21 MHz will receive everything from DC to 21 MHz. Second, it offers reasonable attenuation to IF. It comes at the cost of dropping the 28 MHz band. (We can add an 'extra band' with relays that provides a BPF based narrow band coverage of any one other band (the diode mixer will mix from 144 MHz to 28 MHz).
3. Post IF amp
The diode mixer needs a robust termination to work well and the crystals of 24 MHz were quite lossy. Both these factors lead to adding of a post-mix amplifier ahead of the crystal filter. I know, it is kinda 'old world'. But look : sensitivity is up, crystals need not be expensive.
4. IF amp - not really required
Given that we have gain ahead of the crystal filter, we really dont' need much gain before the audio detector. Hence, just an emitter follower to buffer the signal from the crystal filter to the audio detector.
5. Simpler audio system
I replaced the three-transistor W7EL style audio preamp with an old fav from the BITX20 using a single transistor. The power audio amplifier is no longer the discrete power amp. Instead, I am using a TDA2822 power amp chip. The original Minima audio needed more gain. This chip has 40db of voltage gain.
6. Improved tuning
Each time I used the original Minima tuning i wanted to kill the guy who wrote it. I rewrote the tuning system. Now, it tunes like a normal tuning knob for 100 KHz in 100 Hz steps. However when u hit the band edges it starts to 'scan' first in 20 Khz steps, then 100 KHz and finally in 500 KHz steps. It works well. I must add some visual alert when it starts to scan. But that apart, the system is quite workable.
I am attaching a very rough and incomplete picture of the circuit from my lab notes that shows the changes.
- farhan
No virus found in this message.
Checked by AVG - www.avg.com <http://www.avg.com>
Version: 2015.0.5941 / Virus Database: 4347/9832 - Release Date: 05/21/15
